The field of the invention is oxide-boride-silicon nitride ceramics where addition of silicon nitride to the boride/oxide phases eliminates cracking during formation of the body and increases wear resistance and impact shock resistance for use as a durable wear material, interfacing material and as a cutting tool insert.
Historically, cutting tool inserts were fashioned from metals. As higher speeds of cutting (&gt;500 surface feet per minute, (sfm)) were achieved, metals proved less than satisfactory due to their tendency to chemically react with the workpiece. Nitride, carbide, and oxide coatings were applied to cutting tool metal inserts, however, this improvement is a function of the integrity of the metal coating and eventually wears to the metal substrate.
Generally, cutting tool inserts are function and/or workpiece specific. Inserts for turning are not used as milling inserts and vice versa. The technologies have tended toward dichotomization with ceramic turning tool insert compositions varying significantly from milling tool insert compositions. The ceramic tool milling inserts have been based in the Si.sub.3 N.sub.4 technology while ceramic tool turning inserts have been based in the Al.sub.2 O.sub.3 technology. Herein, the marriage of these technologies produces a ceramic tool insert which exhibits surprising durability characteristics for use as either a turning or milling tool insert.
The following prior art exposes the extent of the ceramic turning and milling tool insert dichotomy. Typical ceramic turning tool insert compositions have been a sintered Al.sub.2 O.sub.3 body with various modifiers being added to increase durability and hardness. U.S. Pat. No. 4,063,908 discloses the addition of TiO.sub.2 and TiC to the Al.sub.2 O.sub.3 sintered ceramic body.
U.S. Pat. No. 4,204,873 discloses the addition of WC and TiN to the Al.sub.2 O.sub.3 sintered ceramic body. U.S. Pat. No. 4,366,254 discloses the addition of ZrO.sub.2, TiN or TiC, and rare earth metal carbides to the Al.sub.2 O.sub.3 ceramic body. U.S. Pat. No. 4,543,343 discloses addition of TiB.sub.2 and ZrO.sub.2 to the Al.sub.2 O.sub.3 ceramic body.
U.S. Pat. No. 3,843,375 departed from the Al.sub.2 O.sub.3 ceramic body disclosing an admixture of TaN and ZrB.sub.2 as the sintered ceramic body.
Typical ceramic milling tool inserts are a Si.sub.3 N.sub.4 sintered ceramic body modified with various metal oxides. U.S. Pat. No. 4,434,238 discloses a Si.sub.3 N.sub.4 body modified with SiO.sub.2 and Y.sub.2 O.sub.3. U.S. Pat. No. 4,264,548 discloses a Si.sub.3 N.sub.4 body modified with SiO.sub.2, Y.sub.2 O.sub.3, and Al.sub.2 O.sub.3.
Generally, there are two tests performed on cutting tool insert material to screen their potential for insert use. These are the turning test and the milling test (or shock test). The turning test looks to durability and is a function of the cutting speed measured in surface feet per minute (sfm). Typically, cutting speeds for the industry average 500 sfm. The milling test looks to the toughness of the material and is measured by the number of cuts as a function of flank wear. The advantage of ceramic inserts is their durability and availability of use at much higher surface feet per minute, an order of magnitude larger. The disadvantage has been low reliability since ceramic materials have tended to chip due to insert tip fracture.
There is, therefore, a need to develop ceramic substrates which exhibit a greater degree of toughness to overcome the reputation for unreliability and a ceramic composition which is capable of embracing both turning and milling functions.